The present invention relates generally to the production of wallboard building panels, and more specifically to an improved gypsum wallboard panel that features a layer of supporting mesh for improving production of the panels, and also for providing increased nail pull resistance.
Gypsum building panels offer a high performance product for a reasonable price for finishing of building spaces. Gypsum, also known as calcium sulfate dihydrate, is heated to drive off crystalline water to produce calcium sulfate anhydrite and/or calcium sulfate hemihydrate, also known as stucco, calcined gypsum or Plaster of Paris. The building panels are made by creating a slurry, combining dry stucco with water. Calcined gypsum and water are combined and an interlocking matrix of gypsum crystals is formed. After the hydration of the calcined gypsum, excess water is driven off by heating, the resulting product is a relatively strong panel, having a good surface for receiving decorative finishes such as paint or wallpaper.
One method of controlling the density of the product is by the addition of a soap-based foam to the liquid slurry. The stucco then sets around the foam bubbles, creating voids in the gypsum matrix. It is important to control the size of the bubbles to avoid undesirable properties in the panels. If the bubbles are too small, a large number of small bubbles are needed to effect the change in density. Where there are lots of bubbles in a confined space, the resulting gypsum matrix has a low compressive strength. Bubbles that are too large cause a decrease in strength and form unsightly blisters under the facing paper.
It is well known to fabricate wallboard or drywall building panels using a sheet of face paper, a slurry of gypsum, and a sheet of backing paper. These components are assembled sequentially on a moving conveyor belt. Reduction in the amount of water needed to produce gypsum is also desirable. Water in excess of that needed to hydrate the calcined gypsum is removed by kiln drying. Fuel costs to operate the drying kiln make it advantageous to reduce the amount of water in a gypsum slurry, while maintaining similar flow characteristics.
Many conventional techniques for reducing the percentage of water in the slurry have produced wallboard with reduced strength, measured as Nail Pull Resistance. In other words, the resulting panel is less resistant to being pulled back over the head of a nail driven into a supporting stud, and as such is less securely retained to the underlying building frame. U.S. Pat. No. 8,566,041, incorporated by reference, discloses a system for calculating Nail Pull Resistance.
Further, it is known to produce wallboard panels with multi-layer gypsum construction, including a first gypsum layer located adjacent the face paper and having a first density, and a second gypsum layer applied upon the first layer. The second layer has a second density, and the density of the second layer is lower than that of the first layer. The higher density of the first layer, achieved by the injection of less foam or water as discussed above, has been found to bond more securely to the face paper. An alternative technique is to differentiate the first and second layers by the addition of different amounts of starch to the slurry, with the first layer receiving more starch than the second layer, to increase strength. During the production of such wallboard panels, it is not uncommon for the flowing of the second layer upon the first layer on the moving production line to cause at least a partial “washout” of the first layer, due to the relatively high velocity of the output of the second layer from the mixer, compared to the conveyor belt line speed. In one example, the output velocity of the slurry forming the second layer is one and a half to three times the speed of the conveyor line. This “washout” disrupts, misaligns or otherwise interferes with the proper positioning and formation of the first layer, which ultimately results in substandard panels.
Accordingly, there is a need for an improved wallboard production system resulting in reduced “washout” and ultimate wallboard panels having increased Nail Pull Resistance.